1. Field of the Invention
The present invention relates to a manufacturing method for a boundary acoustic wave device preferably for use in a resonator, a bandpass filter, or the like, for example, and the boundary acoustic wave device, and also relates to a manufacturing method for a boundary acoustic wave device, which includes a process for adjustment of a frequency, and the boundary acoustic wave device, in more detail.
2. Description of the Related Art
In recent years, in place of surface acoustic wave devices, boundary acoustic wave devices have been drawing attention. In the boundary acoustic wave device, boundary acoustic waves propagate through a boundary between first and second media made of solid substances. Accordingly, in the boundary acoustic wave device, compared with a surface acoustic wave device, it is possible to promote the downsizing of the device. In addition, it is possible to achieve the simplification of a package structure.
When the boundary acoustic wave device is used as a filter or a resonator, it is necessary to set the frequency characteristics thereof with a high degree of accuracy. However, in the boundary acoustic wave device, an interdigital transducer (IDT) electrode exists at the boundary of the first and second media. Accordingly, unlike the surface acoustic wave device, it is difficult to adjust the frequency thereof.
In WO2008/062639 A1, an example of a method of adjusting a frequency of the boundary acoustic wave device is disclosed. As illustrated in FIG. 12, an IDT electrode 103 is disposed at the boundary of a first medium 101 and a second medium 102. The second medium 102 includes a medium layer 102a and a medium layer 102b, and a reforming medium layer 102c is disposed between the medium layers 102a and 102b. 
In WO2008/062639 A1, the outer side portion of the second medium 102 is irradiated with laser light as illustrated by an arrow. Owing to the irradiation of the laser light, the reforming medium layer 102c is heated, and metal included in the reforming medium layer 102c is diffused within the second medium 102. It is supposed that, owing to the diffusion, a reforming portion is formed and hence it is possible to enhance adjustment of a frequency.
In the boundary acoustic wave device, the propagation energy of the boundary acoustic waves is concentrated into a portion near the boundary of the first medium 101 and the second medium 102. Accordingly, in the method of adjusting a frequency described in WO2008/062639 A1, in order to obtain desired frequency characteristics, it has been necessary to form a reforming portion near the boundary of media with a high degree of accuracy.
However, in the method of adjusting a frequency described in WO2008/062639 A1, it is necessary to keep the melting point of the reforming medium layer 102c low or to select material that easily becomes diffuse, as the reforming medium layer 102c, so that media other than the reforming medium layer 102c and the IDT electrode do not become diffuse, and there has been a problem that device design is largely constrained. In addition, in order to reform the reforming medium layer 102c or diffuse metal included in the reforming medium layer 102c into the surrounding media 102a and 102b, it is necessary to cause melting and a solidification action to occur near the melting point of the reforming medium layer 102c. However, if local heating is performed using a focused laser so as to cause the melting and the solidification to occur, the film quality or the diffusion state of the reforming layer becomes non-uniform, and it has been very difficult to adjust a frequency with a high degree of accuracy. Accordingly, it has been difficult to obtain a boundary acoustic wave device including desired frequency characteristics.